1. Field of the Invention
This invention relates to a color printing apparatus that uses a print head for forming dots of a plurality of colors.
2. Description of the Related Art
Serial scan-type printers and drum scan-type printers are dot recording devices which record dots with a print head while carrying out scans both in a main scanning direction and a sub-scanning direction. There is a technique called xe2x80x9cinterlace schemexe2x80x9d, which is taught by U.S. Pat. No. 4,198,642 and Japanese Patent Laid-Open Gazette No. 53-2040, for improving the image quality of printers of this type, especially ink jet printers.
FIG. 25 is a diagram explaining an example of the interlace scheme. In this specification, the following parameters are used to define a printing scheme.
N: Number of nozzles;
k: Nozzle pitch [dots];
s: Number of scan repeats;
D: Nozzle density [nozzles/inch];
L: Sub-scanning amount [dots] or [inch];
w: Dot pitch [inch].
The number of nozzles (N) is the number of nozzles actually used to form dots. In the example of FIG. 25, N=3. The nozzle pitch (k) is the interval between the centers of the recording head nozzles expressed in units of the record image pitch (i.e., dot pitch (w)). In the example of FIG. 25, k=2. The number of scan repeats (s) is the number of main scans in which all dot positions on a main scanning line are serviced. In the example of FIG. 25, s=1, i.e., all dot positions on a main scanning line are serviced in a single main scan. When s is 2 or greater, the dots are formed intermittently in the main scanning direction. This will be explained in detail later. The nozzle density (D) is the number of nozzles per inch in the nozzle array of the print head. The sub-scanning amount (L) is the distance moved in one sub-scan. The dot pitch (w) is the pitch of the dots in the recorded image. In general, it holds that w=1/(Dxc2x7k), k=1/(Dxc2x7w).
The circles containing two-digit numerals in FIG. 25 indicate dot recording positions. As indicated in the legend, the numeral on the left in each circle indicates the nozzle number and the numeral on the right indicates the recording order (the number of the main scan in which it was recorded).
The interlace scheme shown in FIG. 25 is characterized by the configuration of the nozzle array of the recording head and the sub-scanning method. Specifically, in the interlace scheme, the nozzle pitch k indicating the interval between the centers of adjacent nozzles is defined as an integer at least 2, while the number of nozzles N and the nozzle pitch k are selected as integers which are relatively prime. Two integers are xe2x80x9crelatively primexe2x80x9d when they do not have a common divisor other than 1. Further, sub-scanning pitch L is set at a constant value given by N/(Dxc2x7k).
The interlace scheme makes irregularities in nozzle pitch and ink jetting feature to thin out over the recorded image. Because of this, it improves image quality by mitigating the effect of any irregularity that may be present in the nozzle pitch, the jetting feature and the like.
The xe2x80x9coverlap schemexe2x80x9d, also known as the xe2x80x9cmulti-scan schemexe2x80x9d, taught for example by Japanese Patent Laid-Open Gazette No. 3-207665 and Japanese Patent Publication Gazette No. 4-19030 is another technique used to improve image quality in color ink jet printers.
FIG. 26 is a diagram for explaining an example of the overlap scheme. In the overlap scheme, 8 nozzles are divided into 2 nozzle sets. The first nozzle set is made up of 4 nozzles having even nozzle numbers (left numeral in each circle) and the second nozzle set is made up of 4 nozzles having odd nozzle numbers. In each main scan, the nozzle sets are each intermittently driven to form dots in the main scanning direction once every (s) dots. Since s=2 in the example of FIG. 26, a dot is formed at every second dot position. The timing of the driving of the nozzle sets is controlled so that the each nozzle set forms dots at different positions from the other in the main scanning direction. In other words, as shown in FIG. 26, the recording positions of the nozzles of the first nozzle set (nozzles number 8, 6, 4, 2) and those of the nozzles of the second nozzle set (nozzles number 7, 5, 3, 1) are offset from each other by 1 dot in the main scanning direction. This kind of scanning is conducted multiple times with the nozzle driving times being offset between the nozzle sets during each main scan to form all dots on the main scanning lines.
In the overlap scheme, the nozzle pitch k is set at an integer at least 2, as in the interlace scheme. However, the number of nozzles N and the nozzle pitch k are not relatively prime, but the nozzle pitch k and the value N/s, which is obtained by dividing the number of nozzles N by the number of scan repeats s, are set at relatively prime integers instead.
In the overlap scheme, the dots of each main scanning line are not all recorded by the same nozzle but by multiple nozzles. Even when the nozzle characteristics (pitch, jetting feature etc.) are not completely uniform, therefore, enhanced image quality can be obtained because the characteristics of the individual nozzles is prevented from affecting the entire main scanning line.
Mechanical error arising during sub-scanning feeding tends to accumulate with repeated sub-scanning feed operations. In an interlaced scheme, there are cases in which a plurality of sub-scanning feeds is effected during printing of two adjacent raster lines. At this time, the accumulated error of the sub-scanning feed operations gives rise to some variation in the spacing between the two raster lines. Portions in which there is a large such variation show up as banding in the main scanning direction, degrading the image quality.
To reduce banding, in recent years various dot printing schemes have been proposed which include ideas relating to sub-scanning feed amounts. However, not much consideration has been given to the relationship of the location of the banding with respect to dots of different inks, that is, with respect to the location at which the accumulated feed error arises. Thus, there is overlapping of the positions at which banding of dots of different inks occurs, degrading the image quality.
Accordingly, an object of the present invention is to provide a technology for improving image quality by adjusting the relationships with the positions at which the accumulated error of sub-scanning feeds is manifested with respect to dots of different inks.
The present invention uses a print head including a plurality of dot formation element groups for forming dots of different inks where the plurality of dot formation element groups are arrayed in a prescribed order in the sub-scanning direction. The dot formation elements of each group are arranged at an identical pitch k in the sub-scanning direction. The pitch k is set at an integer multiple value that is at least two times a pitch of dots formed on the print medium in the sub-scanning direction. Dot forming is executed while using a mutually equal number N of dot formation elements of each group arrayed at the pitch k where N is an integer of at least 2. The N dot formation elements of each group are selected so that a spacing between the groups of the N dot formation elements is M times the pitch k where M is an integer of at least 2.
In accordance with this invention, since the spacing between adjacent groups is set to be Mxc3x97k where M is an integer of at least 2, the positions at which the accumulated error of sub-scanning feeds is manifested with respect to the dots of the different inks do not always coincide.
The print head may be formed so that a spacing between end dot formation elements of adjacent groups is M times the pitch k where M is an integer of at least 2.
The term xe2x80x9cend dot formation elementxe2x80x9d of each group means the endmost element of the implemented dot formation elements of each group, including working and non-working dot formation elements. Using the above type of print head makes it possible to set the spacing between adjacent groups of dot formation elements at Mxc3x97k where M is an integer of at least 2, even when using close to all of the elements provided on the print head.
The integer M may be set at a value other than (Nxc3x97n+1) where n is an arbitrary integer of at least 1. This makes it possible to more reliably prevent the accumulated feed errors relating to the dots of each ink from showing up at the same position.
The sub-scanning may be performed in accordance with an interlacing scheme where a plurality of sub-scan feeds are sometimes executed between two main scanning passes for forming dots on certain two adjacent main scanning lines. In this type of interlace scheme, the accumulated sub-scanning feed error between the two adjacent main scanning lines can easily become large. With such an interlace scheme, the image-improvement effect obtained by reducing banding by ensuring that the accumulated feed errors for the dots of each ink do not always coincide is even more pronounced.
Specific aspects of the invention can be applied to various types of printing apparatus, printing methods and computer program products.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.